1. Field of the Invention
The present invention relates generally to molecular electronic devices that can be utilized for memory storage, logic circuitry or signal routing. More specifically, the present invention relates to improved methods for making such devices wherein the critical dimensions of the devices are measured in nanometers.
2. Description of Related Art
Molecular electronic devices have been demonstrated to be capable of performing many of the same tasks that are commonly performed by semiconductor (e.g. silicon, gallium arsenide, etc.) devices. These tasks include signal rectification, signal switching and simple logic functions. Such devices are described in: xe2x80x9cMolecular Wire Crossbar Logicxe2x80x9d (U.S. Ser. No. 09/282,045); xe2x80x9cMolecular Wire Crossbar Interconnectxe2x80x9d (U.S. Ser. No. 09/280,225); xe2x80x9cDemultiplexer for a Molecular Wire Crossbar Networkxe2x80x9d (U.S. Ser. No. 09/282,049); xe2x80x9cChemically Synthesized and Assembled Electronic Devicesxe2x80x9d (U.S. Ser. No. 09/292,767); and xe2x80x9cElectrically Addressable Volatile and Non-Volatile Molecular Based Switching Devices (U.S. Ser. No. 09/459,246). Molecular electronic devices are also described in U.S. Pat. Nos. 6,128,214 and 6,159,620.
An advantage of molecular electronic devices is that the device performance characteristics originate from molecular properties. This has several implications. First, it means that the devices can potentially scale down in size to nanometer dimensions without significant change in device performance. Second, it also means that the unique electronic properties that can be designed into molecular structures can also be designed into solid state devices. These advantages are not characteristic of semiconductor devices. However, many molecular electronic devices that have been fabricated to date involve fairly awkward device processing steps. As one example of this awkward processing, electrical connections to the molecules are often evaporated through contact shadow masks, meaning that a thin metal foil that has been previously patterned with holes of various shapes is placed in contact with the molecular thin film and metal electrodes are deposited by directing a metal vapor through the open pattern. This technique has serious limitations in terms of the size resolution and complexity of electrode patterns that can be deposited. For example, it is very difficult and expensive to fabricate shadow masks that have patterned features that are smaller than a couple of micrometers in size.
As a second example of the current art that is utilized for molecular electronic devices, a nanometer scale wire is defined using electron beam lithography and great effort is made to fabricate a wire that has a very thin cross-section. The wire is then xe2x80x9cbrokenxe2x80x9d in a manner similar to how a fuse is blown. Under appropriate conditions, the broken junction can be designed to have a gap that is of molecular dimensions, so that molecules can be chemically attached to bridge across the junction. Once again, while it is possible to make electrical contact to the molecules in this way, the device processing steps are just awkward, and the performance characteristics of such a device are difficult to reproduce across many devices.
Nevertheless, the above-described procedures have been developed because, while at thin film of organic molecules may have desirable characteristics for electronic device applications, it is also an inherently delicate film. This is because such a film may melt, flow, or be otherwise damaged by low-temperature processing steps such as the spin-coating of a photolithographic resist materials. A set of processing techniques for dealing with such films have not been developed. It would be desirable to provide improved processing techniques that would eliminate the problems associated with existing processing technology and allow the production of molecular electronic devices having electrode patterns with nanometer scale dimensions.
DEFINITIONS. The following definitions apply to the present invention:
xe2x80x9cMol-RAMxe2x80x9d in this context refers to molecular-switch based array of memory cells.
xe2x80x9cMolecular electronic devicesxe2x80x9d in this context refers to devices in which some critical component of the device, such as the wire or the switch, is a molecule or a collection of molecules.
A xe2x80x9cmemory bitxe2x80x9d in this context refers to a physical device that can exist in two states (xe2x80x980xe2x80x99 or xe2x80x981xe2x80x99) that can be distinguished from one another by electrically probing the device.
xe2x80x9cLithographic processingxe2x80x9d in this context refers to any procedure in which light or electron beams are used to produce a chemically or materially differentiated pattern onto a substrate.
A xe2x80x9cswitchxe2x80x9d in this context refers to a physical device that can switch between two states, such as xe2x80x98openxe2x80x99 and xe2x80x98closed,xe2x80x99 and the difference between the two states can be probed electrically. The difference between the two states of a switch is typically greater than for a memory bit. For example, if the electrical property that is measured to determine the state of the switch is the resistance of the device, then a memory bit may be characterized by a 20% change in resistance, while a switch may be characterized by a 200% change in resistance. A switch can be used as a memory bit, but a memory bit may not necessarily be useful as a switch.
xe2x80x9cSelf-assembledxe2x80x9d in this context refers to a system that naturally adopts some geometric pattern because of the identity of the components of the system; the system achieves at least a local minimum in its energy by adopting this configuration. For example, a self-assembled molecular monolayer is a geometrically arranged monolayer film of molecules that is formed when certain molecules chemically bind to a certain surface. The organization of such a self-assembled monolayer is controlled by both the geometric registry of the molecules with the atomic structure of the underlying surface, as well as the interactions between neighboring molecules in the monolayer.
xe2x80x9cSingly configurablexe2x80x9d in this context means that a switch can change its state only once via an irreversible process such as oxidation or reduction reaction; such a switch can be the basis of a programmable read only memory (PROM), for example.
xe2x80x9cReconfigurablexe2x80x9d in this context means that a switch can change its state multiple times via a reversible process such as an oxidation or reduction; in other words the switch can be opened and closed multiple times, e.g., the memory bits in a random access memory (RAM).
A xe2x80x9ccrosspoint memoryxe2x80x9d in this context means a memory circuit that consists of a grid of crossed wires. At each junction of the grid is a memory bit, in which some material, such as switching molecules, are sandwiched between the electrodes. The xe2x80x980xe2x80x99 or xe2x80x981xe2x80x99 state of the memory bit may be set electrically, and that state of the memory bit may be probed electrically. The electrical setting or probing of the bit is carried out by electrically addressing the two wires of the crosspoint memory that form the intersection.
xe2x80x9cRedox activexe2x80x9d in this context means that a molecule or molecular junction can be electrochemically reduced or oxidized, meaning that electrical charge can be added or taken away from the molecules or molecular junction.
A xe2x80x9cwetting filmxe2x80x9d in this context refers to a film that completely and uniformly covers another film. This term does not imply that the wetting film is liquid, it only refers to how the wetting film coats an underlying substrate. If a top material does not uniformly wet a bottom material, then that top material will instead form islands and patches of coverages.
xe2x80x9cMicron-scale dimensionsxe2x80x9d refers to dimensions that range from 1 micrometer to a few micrometers in size.
xe2x80x9cSub-micron scale dimensionsxe2x80x9d refers to dimensions that range from 1 micrometer down to 0.04 micrometers.
xe2x80x9cNanometer scale dimensionsxe2x80x9d refers to dimensions that range from 1 nanometers up to 50 nanometers (0.05 micrometers).
xe2x80x9cMicron-scale wiresxe2x80x9d and xe2x80x9csubmicron-scale wiresxe2x80x9d refers to rod or ribbon-shaped conductors of semiconductors with widths or diameters having the dimensions of 1 to 10 micrometers, heights that can range from a few tens of nanometers to a micrometer, and lengths of several micrometers and longer.
xe2x80x9cHysteresisxe2x80x9d in this context refers to the character of a current-voltage measurement such that the forward voltage trace (negative to positive voltage) is characterized by a different current flow than the reverse voltage trace (positive to negative voltage). VLH refers to the low voltage end of the hysteresis loop, and VHH refers to the high voltage end of the current loop. VMH is a voltage value somewhere between VLH and VHH.
xe2x80x9cNon-destructive readxe2x80x9d in this context refers to the operation of a memory cell such that the information in the cell can be read out (accessed) without affecting the status of the memory bit.
A xe2x80x9cshortxe2x80x9d or a xe2x80x9cshorted devicexe2x80x9d in this context refers to an unintended fixed electrical connection between various components of a device, or between various devices.
A xe2x80x9ccircuitxe2x80x9d in this context refers to several interconnected devices that together perform some task, such as a logical operation, memory storage, or signal routing.
In accordance with the present invention, a method is provided for making molecular electronic devices which is an improvement over existing fabrication techniques. The method is suitable for producing molecular electronic devices where the various electronic structures have micron to submicron to nanometer scale dimensions.
The method involves providing a substrate having a surface on which is located a first electrode pattern. Bistable switching molecules, or some other molecules with a desirable electronic characteristic, are deposited onto the substrate surface to form a molecular layer that covers the substrate surface including the first electrode pattern. As a feature of the present invention, an electrically conductive material is deposited onto the molecular layer to form an electrically conductive protective layer that serves to protect the underlying molecular layer. The protective layer allows one to use lithography or other conventional patterning techniques, such as stamping or imprinting, to form a second electrode pattern on the surface of the protective layer without damaging the underlying molecules. The ability to use lithography and other related patterning techniques makes it possible to fabricate complex circuit patterns and to form a second electrode pattern above the first that has dimensions that are only limited by the lithography or patterning approach.
The second electrode pattern is formed on the protective layer so that the second electrode pattern overlaps the first electrode pattern to form at least one electrode intersection. The protective layer is then removed at the locations which remain exposed after formation of the second electrode pattern to form at least one electrode intersection where the molecular switching layer and electrically conductive protective layer are sandwiched between the first and second electrode patterns. The electrically conductive protective layer at each intersection forms an integral part of the second electrode. Selective removal of the conductive protective layer from those areas that are not located under the second electrode pattern is necessary in order to limit electrical conductivity to the electrodes and electrode intersections. Without this step, an electrical short might exist between the top and bottom electrodes, and if a circuit has been fabricated, then the various devices within the circuit might also be shorted to one another.
The present invention is not only directed to a method for making molecular electronic devices, but also covers the devices themselves. Such devices include a substrate having a surface on which is located a bottom electrode pattern having an interior surface. A top electrode pattern having an interior surface is provided wherein the top electrode pattern overlaps said bottom electrode pattern to form at least one electrode intersection located between the interior surfaces of the first and second electrode patterns. Sandwiched between these two electrodes are two layers. The lower layer, which consists of molecules that have unique electrical properties, such as a bistable switching characteristic, is situated on top of the bottom electrode. The upper layer is an electrically conductive protective material, and this layer provides an interface between the molecular layer and the top electrode pattern. The resulting device includes one or more electrode intersections or switches wherein the molecules and electrically conductive protective material are sandwiched between the first and second electrode patterns.
The present invention also is directed to the intermediate assemblies that are fabricated during the various steps of the manufacturing process. For example, the invention covers the assembly prior to formation of the second electrode pattern on the electrically conductive protective layer. The assembly includes a substrate comprising a surface on which is located a first electrode pattern. A layer of molecules, such as bistable switching molecules, is provided which has been deposited onto the substrate surface to form a molecular layer that covers the substrate surface including the first electrode pattern. The assembly further includes a layer of electrically conductive material that has been deposited onto the molecular switching layer to form an electrically conductive protective layer. This protective layer has an exposed surface on which the second electrode pattern may be formed by lithograhy or other high resolution technique used for fabricating electrode patterns having dimensions that are limited only by the lithographic technique, and thus may be scaled to nanometer dimensions.
The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings.